Optical element with high scratch resistance

ABSTRACT

An optical element is provided that includes a substrate that is transparent in the visible spectral region and a multilayer anti-reflection coating on the substrate. The coating has alternating layers of layers having a first refractive index and of layers having a second, higher refractive index. The layers having the higher refractive index contain nitride or oxynitride and the layers having the first refractive index contain oxide of silicon and of at least one other element. The molar fraction of silicon in the layers having the first refractive index is predominant when compared to the molar fraction(s) of the other element or elements. The uppermost layer of the coating is a layer having the first refractive index. A layer of chain-form organofluoro molecules is disposed on the coating, wherein the molecules are bonded at the ends to the surface of the optical element.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. §119(a) of German PatentApplication No. 10 2014 108 058.0 filed Jun. 6, 2014, the entirecontents of which are incorporated herein by reference.

BACKGROUND

1. Field of the Disclosure

In general, the invention relates to optical elements, such as viewingpanels or cover glasses for optical displays, for example, of mobilephones or watches, or for optical systems, such as, for example,objectives or optical measuring instruments. In particular, theinvention relates to measures for improving the stability of the surfacerelative to scratching.

2. Description of Related Art

In order to protect surfaces of optical elements against scratching, itis known to provide the surface with a hard coating. Thus, a hardanti-reflection coating is known from EP 2492251 A1.

However, it has been determined by the present disclosure that there isa continuing need for coatings with improved scratch resistance. Thus,the object of the invention is now based on improving the scratchresistance of such coatings.

SUMMARY

Accordingly, the invention provides an optical element with high scratchresistance with a substrate that is transparent in the visible region ofthe spectrum, in particular, in at least a partial region of thewavelength region from 380 nanometers to 780 nanometers. A multilayeranti-reflection coating is deposited on the substrate, this coatinghaving alternating layers of layers having a first refractive index andof layers having a second, higher refractive index, wherein the layershaving the higher refractive index contain nitride or oxynitride and thelayers having the first refractive index contain oxide of silicon and ofat least one other element, wherein the molar fraction of silicon ispredominant when compared with the molar fraction(s) of the otherelement or elements in the layers having the first refractive index, andwherein the uppermost layer of the multilayer anti-reflection coating isa layer having the first refractive index, and wherein a layer ofchain-form organofluoro molecules is disposed on the uppermost layer,wherein the molecules are preferably bonded at their ends to the surfaceof the uppermost layer of the anti-reflection coating. Due to thebonding to the surface at their ends, the chains of the organofluorocoating are oriented away from the surface, or have a preferredorientation along the surface normal line.

In the sense of the invention, an optical element is scratch-resistant,if, after an abrasively acting stress of the coating, scratches aresurprisingly small in comparison to the uncoated substrate, at leastvisually, or are not visible at all. The layers containing nitride oroxynitride are also preferably silicon-based like the layers having thefirst refractive index. In particular, the layers having the secondrefractive index, just like the layers having the first refractiveindex, can be doped with at least one other element. According to thisembodiment, the layers having a higher refractive index contain nitrideor oxynitride of silicon and of at least one other element.

It is likewise also possible, however, to form the layers having thesecond, higher refractive index from nitrides or oxynitrides of elementsother than silicon. For example, layers containing nitrides oroxynitrides of titanium or chromium as the only component or as theprincipal component shall be named.

It will not be excluded that still other intermediate layers are alsofound between the alternating layers having the first and the secondrefractive indices. For example, doped silicon oxynitride layers couldlikewise be interposed between layers having the first and the secondrefractive indices, these layers containing silicon oxide or nitride,respectively, preferably containing silicon nitride, if this results ina favorable optical design.

Furthermore, gradients of layers, which vary in oxygen or nitrogencontent, at least partially, are also to be understood as intermediatelayers.

It is shown surprisingly that the organofluoro molecules still furtherconsiderably improve the scratch resistance of the already very hardanti-reflection coating based on the nitride layers. This is expressedby wear tests in a clearly extended period of time until there is asignificant increase of light scattering and a change in theanti-reflection properties.

In particular, the layer of organofluoro molecules is particularlypreferred to be very thin, i.e., monomolecular. This single-moleculelayer is thus not active optically or is just barely active and is alsocorrespondingly visually inconspicuous.

It is believed that the organofluoro layer, although it is not itselfhard, reduces interaction with the surface. In particular, it alsohappens here that the uppermost oxide layer of the anti-reflectioncoating has a hardness that is less than that of the nitride layerfollowing thereon based on the alternating sequence of layers. In thiscase, an effect is based on a reduction in the friction coefficient ofthe surface due to the organofluoro molecules.

The organofluoro layer prevents the formation of chemical bonds betweenthe surface and an abrasive medium, as is produced during an effectiveabrasion. Thus, glass is readily polished with cerium oxide, since thelatter compound forms covalent bonds with glass and thus clearlyincreases the material removed. Abrasion and polishing typicallyrepresent a physical-chemical process.

The coating of the optical element is thus able to resist scratching aslong as the organofluoro molecules are bound to the surface. A strippingof the organofluoro molecules may occur, however, if, e.g., the surfaceis chemically attacked through the organofluoro layer or is attacked atsites where the organofluoro layer has broken down. The additionalelement present as an oxide in the uppermost layer comes to bear here.That is, the chemical resistance of the surface can be considerablyimproved by means of this at least one additional element and thedurability of the organofluoro molecules can also be improved indirectlythereby.

According to a particularly preferred embodiment of the invention, thelayer of organofluoro molecules is thus deposited directly onto thesurface of the uppermost oxide layer of the anti-reflection coating.Optionally, however, yet another thin, optically inactive intermediatelayer can also be provided, which serves as an adhesion promoter. Suchan adhesion promoter preferably has a layer thickness of less than 10nm, preferably less than 8 nm, more preferably of less than 6 nm, inorder not to be optically active. A layer containing silicon oxide,particularly a mixed oxide layer with silicon oxide as the principalcomponent, is suitable for such an adhesion promoter layer.

The hard anti-reflection coating with the layer of organofluoromolecules is particularly suitable also for hard substrates with highKnoop hardness and/or a high modulus of elasticity. In this case, a highKnoop hardness is also essentially retained after the coating, or it isimproved even further. Accordingly, according to an embodiment of theinvention, the Knoop hardness on the coated surface of the opticalelement amounts to at least 600 kilograms per square millimeter (600kg/mm2). According to another embodiment, a substrate is selected thathas a modulus of elasticity of at least 50 GPa. One of the preferredmaterials for the substrate is sapphire, or Al₂O₃.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained more precisely in detail below, alsowith reference to the appended drawings.

FIG. 1 shows a cross section through an optical element according to afirst embodiment;

FIG. 2 shows a cross section through a variant of the example shown inFIG. 1;

FIG. 3 shows plots of the spectral reflectance measured on two samplesbefore and after an abrasion test;

FIG. 4 shows bar graphs for the percent increase in the haze value aftera sandpaper test for an uncoated, chemically prestressed glass substrateand for three different coated, chemically prestressed glass substrates;

FIG. 5 shows a diagram for the increase in the haze value and thereflectance dependent on different influence factors;

FIG. 6 shows a diagram for the increase in the haze value and thereflectance dependent on different influence factors;

FIG. 7 shows an additional exemplary embodiment of an optical elements;and

FIG. 8 shows an additional exemplary embodiment of an optical element.

DETAILED DESCRIPTION

FIG. 1 and FIG. 2 show examples of optical elements 1 according to theinvention with high scratch resistance, having a transparent substrate10 in the visible spectral region, and a multilayer anti-reflectioncoating 2 deposited on the substrate 10. Layers 25, 27 of theanti-reflection coating 2 having a first refractive index alternate withlayers 24, 26 having a second, higher refractive index in comparison tothe first refractive index.

A layer 3 of chain-form organofluoro molecules is disposed on theuppermost layer 27 of the anti-reflection coating, wherein the moleculesare bonded at their ends to the surface of the uppermost layer 27 of theanti-reflection coating 2.

The organofluoro molecules preferably contain perfluorinated carbonchains, in which all hydrogen atoms, in particular, can also be replacedby fluorine atoms. In addition, the organofluoro molecules arepreferably individually covalent at the surface of the optical element1. The individual molecules can also enter into more than just onecovalent bond with the surface.

Suitable for layer 3 without limitation to the special exemplaryembodiments are, in particular, perfluoro ethers with terminal silanegroup, for example, the “Optool™ AES4-E” coating or the “Optool™ DSX”coating of Daikin Industries LTD., perfluoro ethers with two terminalsilane groups, for example, the “Fluorolink S10” coating of SolvaySolexis; perfluoro alkyl silane, preferably with purely inorganicsilicon oxide fraction.

Coating is preferably conducted via liquid coating by means of a coatingfluid. Suitable for this purpose is, for example, roll-coating,spin-coating, dip-coating or spraying methods.

According to yet another embodiment of the invention, layer 3 is appliedby a vacuum coating process, wherein the organofluoro molecules arevaporized in vacuum and are deposited on the surface of the substrate 10coated with the anti-reflection coating 2. Suitable, for example, is“Duralon UltraTec” of Cotec GmbH, Karlstein, which is marketed in tabletform.

An adhesion promoter layer between layer 3 and the anti-reflectioncoating 2, as it is provided e.g., according to WO 2012/163946 A1, isnot necessary, in order to obtain the improvement in the scratchresistance of the surface of an optical element 1 according to theinvention.

The layers 24, 26 with higher refractive index essentially containnitride or oxynitride, and the layers with the first refractive indexcontain oxide of silicon and of at least one other element. Siliconrepresents the principal fraction of the nitride or oxide elementspresent, however, in terms of quantity in any case, according to apreferred embodiment, so that the molar fraction of silicon in thelayers is predominant when compared to the molar fraction(s) of theother element or elements. The ratio of the quantity of silicon to thequantity of the other element or elements in the individual layers ofthe anti-reflection coating preferably amounts to at least 5:1,preferably at least 8:1. In other words, at least five times, preferablyat least eight times more silicon is contained in the anti-reflectioncoating 2, compared to the quantity of the at least one other element.

The at least one other element is preferably selected from the elements:aluminum, tin, magnesium, phosphorus, cerium, zirconium, titanium,barium, strontium, cesium, niobium, boron.

These elements form both oxides as well as nitrides and simultaneouslyimprove the chemical resistance of the coating.

According to another embodiment, nitrides or oxynitrides of an elementother than silicon are used as the principal component or, in fact, theonly component for the layers having the second, higher refractiveindex. For example, one thinks of titanium nitride, boron nitride,aluminum nitride, and/or chromium nitride, or of oxynitrides oftitanium, boron, aluminum, and/or chromium.

The improvement in the chemical stability of the coating according tothe invention can be demonstrated, for example, by means of a salt waterspray mist test, preferably according to DIN EN 1096-2:2001-05.

Vacuum deposition methods are particularly suitable for the productionof the anti-reflection coating. Accordingly, in general, the inventionprovides, without limitation to the example shown, a method forproducing an optical element 1 according to the invention, in which

a substrate 10 that is transparent in the visible spectral region isprepared and

a multilayer anti-reflection coating 2 is applied onto the substrate 10,in that

alternating layers of a layer having a first refractive index and alayer having a second, higher refractive index are deposited by means ofa vacuum deposition method, wherein the layers having the higherrefractive index contain nitride or oxynitride and the layers having thefirst refractive index contain oxide of silicon and of at least oneother element, and wherein the molar fraction of silicon is predominantwhen compared with the molar fraction(s) of the other element orelements in the layers with the first refractive index, and wherein theuppermost layer of the multilayer anti-reflection coating is a layerhaving the first refractive index, and wherein a coating with a layer 3of chain-form organofluoro molecules is applied on the anti-reflectioncoating, wherein the molecules are bound on their ends to the surface ofthe optical element, most preferably to the uppermost layer 27 of theanti-reflection coating 2.

In addition, it is preferred that the layers 24, 25, 26, 27 of theanti-reflection coating 2 are deposited by reactive magnetronsputtering. A silicon sputtering target, which is doped with at leastone other element, can be used for this purpose. Aluminum, preferablywith a content of at most 20 mol % in the target is particularlypreferred as the at least one other element. The layers of theanti-reflection coating 2, which are produced by means of such a target,are thus oxide layers having the first refractive index and nitridelayers of silicon and aluminum having the second refractive index, thequantity ratio of silicon to aluminum amounting to at least 5:1. Insteadof aluminum, however, the other named elements may also be present, or,in addition to aluminum, at least one other of the named elements may bepresent.

As an alternative to the deposition by reactive magnetron sputtering,ion-beam sputtering can also be used in order to apply the layers of theanti-reflection coating.

According to a particularly preferred embodiment, on which the exemplaryembodiments of FIG. 1 and FIG. 2 are also based, an anti-reflectioncoating 2 with a multilayer stack is deposited onto the substrate 1,this layer stack being comprised of four successive layers 24, 25, 26,27. The lowermost layer 24 is a higher refracting layer containingsilicon nitride, wherein the other higher refracting layer 26 containingsilicon nitride, which forms the uppermost high-refracting layer of thelayer stack, has the greatest layer thickness within the layer stack,and wherein the uppermost layer 27 of the layer stack forms a layerhaving the lower refractive index and is composed of silicon oxide,preferably with a fraction of aluminum, and has the second-greatestlayer thickness among the layers of the layer stack; wherein the firstlayer 24, plus the second layer 25, which, like the uppermost layer 27,is a layer having the lower refractive index and is composed of siliconoxide with a fraction of aluminum, together have a layer thickness thatis thinner than the layer thickness of the uppermost layer.

In particular, in the example shown in FIG. 2, the uppermosthigh-refracting layer of the likewise four-layer anti-reflection coatingis very thick. The example shown in FIG. 2 is optimized for a very highscratch resistance. The anti-reflection properties, however, aresurprisingly only slightly poorer than in the example optimized for lowreflectance that is shown in FIG. 1.

The coating of the surface 13 of the substrate 10 may also compriseother layers in additional to the anti-reflection coating 2. Accordingto one embodiment, it is generally provided for this purpose, withoutlimitation to the examples shown in the figures, that a layer 28 thatcontains silicon oxide and serves as an adhesion promoter for thesubsequently deposited anti-reflection coating 2 is deposited on thesurface 13 of the substrate 10. The layer 28, together with theanti-reflection coating 2, forms an inorganic coating 20. A thin siliconoxide layer, particularly a layer with a composition like thelow-refracting layers 25, 27 of the anti-reflection coating 2 have, issuitable, for example, for the layer 28.

Such an adhesion promoter layer in the form of a layer 28 composed ofsilicon oxide is particularly suitable for coupling the hardanti-reflection coating with hard substrates. Such a coating isparticularly suitable for a substrate 10 composed of sapphire or Al₂O₃.According to one embodiment of the invention, an optical element 1 isprovided in the form of a watch glass with sapphire substrate 10 and thecoating according to the invention. The coating makes it possible toobtain the scratch resistance of uncoated sapphire, or to at least comeclose to this, but simultaneously to also improve the optical propertieswith respect to reflectance. Sapphire has a high refractive index ofmore than 1.7, so that sapphire glasses are intensely reflective. Thisdisadvantage will be avoided with the anti-reflection coating.

The relative thicknesses of the layers, which were explained above forthe exemplary embodiment of FIG. 1, are also fulfilled in the exampleshown in FIG. 2 and preferably also apply to the embodiment having avery thick uppermost layer 26 having the high refractive index, which isexplained below.

In the variant of FIG. 2, the uppermost layer containing siliconnitride, or the uppermost layer 26 having the second refractive index isclearly thicker than in the example shown in FIG. 1. In general, withoutlimitation to the special example shown, according to one embodiment ofthe invention, it is provided for this purpose that the anti-reflectioncoating 2 comprises a layer stack of four successive layers 24, 25, 26,27 with two layers 24, 26 having the second, higher refractive index, inwhich the upper 26 of these two layers having the second, higherrefractive index has a layer thickness that amounts to at least 40% ofthe thickness of the anti-reflection coating 2, preferably at least 60%of the thickness of the anti-reflection coating 2, more preferably atleast 70% of the thickness of the anti-reflection coating 2.

In the example shown in FIG. 2, the thickness of the layer 26 in factamounts to more than 70% of the layer thickness of the anti-reflectioncoating 2.

According to yet another embodiment of the invention, it is providedthat the anti-reflection coating 2 comprises a layer stack, in which theuppermost layer 26 having the second refractive index has the greatestlayer thickness of all the layers of the anti-reflection coating and hasa layer thickness in the range of 100 nm to 700 nm, preferably of 300 nmto 600 nm, more preferably of 400 nm to 500 nm. This embodiment of theinvention preferably also relates to the examples shown in FIGS. 1 and2, but can also be applied to other anti-reflection coatings with moreor fewer layers. The thickness of the upper nitride hard-material layerprovides for a high resistance capability.

In order to test and to compare the scratch resistance of the opticalelements according to the invention, an abrasion test can be conducted.The test used for the investigations is a modified Bayer test accordingto ASTM F735-11. In this case, the optical elements to be tested arecovered with a granular, abrasive medium in a tub, and the tub is set tooscillate. Aluminum oxide sand with a grain size between 297 μm and 420μm and a Mohs hardness of 9 was used as the abrasive medium. The bathwas filled with an amount of 2 kg of sand, so that a sand layer ofapproximately 18 mm thickness results. The bath oscillates at 150 cyclesper minute.

FIG. 3 shows the spectral reflectance of two optical elements in thevisible spectral region between 450 nm and 700 nm, each measured beforeand after an abrasion test as described above, with 8000 cycles.

Curve “A” is the reflectance of a sapphire substrate coated with afour-layer anti-reflection coating; curve “C” is the reflectance of thissample after the abrasion test.

Curve “B” shows the spectral reflectance of an optical element accordingto the invention with a sapphire substrate, in which the anti-reflectioncoating 2 was coated additionally with a layer of organofluoro molecules3. Finally, curve “D” shows the reflectance of this sample after theabrasion test. As can be seen from the curves, the spectral reflectancechanges less after the abrasion test in the case of the optical elementaccording to the invention (i.e., from curve “B” to curve “D”) than itdoes in the case of the anti-reflection coating without organofluorocoating (curves “A” and “C”). The difference between the curves “A” and“B” lies both in fluctuations in process parameters during coating aswell as in the presence of the organofluoro coating.

The effect of the mechanical stability or the effectiveness with respectto the scratch sensitivity of the coated optical elements according tothe invention can also be conducted with a sandpaper test. The effect ofthe layer system according to the invention will be explained in thefollowing on the basis of chemically prestressed glass elements. In thiscase, the effect of sand grains on the glass elements is simulated.

Measurements were conducted of the percent increase in the haze value asa consequence of the sandpaper test on anti-reflection coatings as shownin FIG. 1 and FIG. 2. Here, a chemically prestressed aluminosilicateglass serves as the transparent substrate 10.

The haze measurement is conducted according to the ASTM StandardD1003-95. In this case, the fraction of scattered light in the lighttransmitted by the glass element is compared with the intensity of thetotal transmitted light.

The scattering is thus a measure for the fraction of the surface damagedby scratches. A defect in the surface of the glass leads to a deflectionof the beam striking perpendicular to the glass surface and is deflectedfrom its direction of incidence. The greater the damage is to thesurface, the more the light is deflected away from the detector. Thus,the haze value, given in percent, is a measure for the degree of damageto the surface.

The results of the haze measurements are shown in FIG. 4 as bar graphs.The measurement values in FIG. 4 thus represent the percent increase inthe fraction of scattered light due to scratches and other damage to thesubstrate surface after the sandpaper test. The sample named “Design 8”has a four-layer structure similar to the example of FIG. 1. In the caseof the sample named “Design 1”, the layer structure corresponds to thefour-layer structure according to FIG. 2 in which the upper layer 26having the second refractive index has a layer thickness thatconstitutes more than 70% of the total layer thickness of theanti-reflection coating 2.

The change in the haze value after the sandpaper test shows that thecoating of the glass substrate with the layer system according to Design1, which is optimized for scratch resistance with a particularly thicklayer for the upper nitride hard-material layer of the anti-reflectioncoating 2, shows a surprisingly clear improvement when compared to theuncoated glass substrate, but also when compared to the layer system ofDesign 8 (according to FIG. 1), whose upper layer 26 having the secondrefractive index of the anti-reflection coating 2 is less than one-thirdas thick as the one according to Design 1.

In addition, it has been shown surprisingly that a layer systemaccording to the invention, in which a layer 3 of organofluoro moleculesis additionally introduced, considerably improves the prevention ofscratches. As can be seen from FIG. 4, the measured increase in the hazevalue is reduced once more by a factor of three when compared to thehard anti-reflection coating without layer 3, and amounts to only 0.2%.In contrast to this, the increase in the haze value in the case of theuncoated, chemically prestressed aluminosilicate glass reference sampleat 21.8% is greater by a factor of more than 100.

In the following, it will be explained on the basis of FIG. 5 and FIG. 6which effects individual influence factors have on the scratchresistance. FIG. 5 and FIG. 6 show diagrams of the increase in the hazevalue (named “haze_Jiff”) and of the increase in reflectance (named“R_diff”) due to an abrasion test (modified Bayer test with 8000abrasion cycles) as a function of the different influence factors. Thediagrams were created by comparison of a plurality of samples, each witha different material for substrate 10, with and without anti-reflectioncoating 2, with and without chemical prestressing, as well as with andwithout layer 3 of organofluoro molecules.

In this way, the effects of the individual influence factors can beisolated. In FIG. 5 each of these influence factors is evaluated onoptical elements without layer 3; in FIG. 6, with an applied layer 3.Here, the scale on the abscissa indicates for which fraction theinfluence factor is present. Each time, four diagrams are presented nextto one another; from left to right, the influence factors are: substratematerial (diagrams designated “material”), chemical prestressing(diagrams designated “chem.str”, anti-reflection coating 2 (diagramsdesignated “AR” and organofluoro layer 3 (diagrams designated “FOC”.Here, except for the “material” diagrams, the value “0” represents theabsence of the influence factor and the value “1” represents itspresence. In the “material” diagrams, the value “0” denotes aborosilicate float glass and the value “1” denotes an aluminosilicateglass. Reflectance involves the average value in the wavelength regionfrom 380 to 780 nm.

With the organofluoro coating, the reflectance increases on average by0.55% in the abrasion test. Without the organofluoro coating, thisincrease due to abrasion is 0.61%. That is, the increase in reflectancecaused by abrasion is reduced by 10% due to the organofluoro layer 3.

With the organofluoro coating, the haze (the light scattering) increaseson average by 0.037% in the abrasion test. Without a layer 3, thisincrease amounts to 0.051% due to abrasion. This means that the increasein haze caused by abrasion is reduced by 27% due to layer 3.

In an evaluation, if all samples are considered independently from otherparameters, one does not observe a comparison relative to the influencestrength of the other parameters, but only the influence of theorganofluoro layer, and one arrives at the following values: With theorganofluoro layer 3, the reflectance of all samples increases onaverage by 0.527% in the abrasion test. Without the organofluorocoating, this increase due to abrasion is 0.565%. That is, the increasein reflectance caused by abrasion is reduced by 7% due to theorganofluoro coating. With the organofluoro coating, the haze (the lightscattering) increases on average by 0.025% on all samples in the harshabrasion test. Without the organofluoro layer 3, this increase due toabrasion is 0.041%. That is, the increase in haze caused by abrasion isreduced by 38% due to the organofluoro layer 3.

Based on the diagrams (FIG. 5 and FIG. 6), it is observed that theinfluence on abrasion resistance (change in the reflectance or haze dueto abrasion) depends greatly on the material, prestressing, and theanti-reflection coating. Nevertheless, the influence of the organofluorolayer 3 is clearly present.

Furthermore, it has been shown, however, that the combination of theorganofluoro layer 3 with the anti-reflection coating 2 results in aclearly greater effect than the two influence factors by themselves.Thus, the effect in the case of the impact of a sandpaper test shown inFIG. 4 is essentially greater. The anti-reflection coating 2 alonebrings about a reduction in the haze increase by a factor of 30. Withlayer 3, this effect is once more considerably increased and bringsabout a reduction by a factor of 100. The effect caused only by theorganofluoro layer 3 according to FIGS. 5 and 6 (modified Bayer testwith 8000 cycles) is essentially smaller, even if it is considered thatthe abrasion test of FIGS. 5 and 6 simulates clearly harsher conditionsand the results therefore cannot be directly compared.

The fact that an interaction between layer 3 and the anti-reflectioncoating 2 according to the invention is present is also clear accordingto the following table:

Haze increase Haze increase Improvement in the with layer 3 withoutlayer 3 haze increase Without AR 0.04 0.07 43% With AR 0.06 0.3 80%

The table shows the effect of layer 3 for a borosilicate float glasssubstrate 10 with and without anti-reflection coating 2 based on thepercent improvement in the increase of the haze value according to theabrasion test. A high percent improvement in this case means that theincrease in the haze value is low when compared to the sample prior tothe abrasion test. Accordingly, layer 3 reduces the haze increase whencompared to the uncoated substrate.

In contrast, however, if a combination of anti-reflection coating 2 andorganofluoro layer 3 according to the invention are used, once again anessentially clearer improvement in the increase of light scattering(haze) is observed when compared to a specimen coated withanti-reflection coating 2, but without layer 3. The improvement in theincrease of the haze value is, at 80%, almost twice as much as that forthe uncoated sample. In the case of the increase in reflectance, theeffect is in fact smaller (see FIG. 3), but still clear. Here, it shouldalso be noted that in the case of an anti-reflection coating thereflectance is sensitive to a reduction in the layer thickness of theuppermost layer 27. If the layer thickness is reduced by abrasion, thenthis leads to a spectral shift of the interference effects. In contrast,in the case of an uncoated glass, the refractive index and along withit, the reflectance, does not change in the case of an abrasion. In thisrespect, the improvement obtained by layer 3 in the case of theanti-reflection coating, which is more sensitive relative to reflectancewith abrasion, also shows a very clear effect.

The over-proportionally large effect for the improvement in the scratchresistance is attributed to an interaction between the doping of theuppermost layer 27 of the anti-reflection coating and the coating bymeans of the organofluoro molecules of layer 3. On the one hand, thechemical resistance is increased by the at least one other oxide inaddition to silicon oxide. This oxide of another element possibly alsohas an influence on the covalent binding of the organofluoro moleculesto the surface of the uppermost layer 27 of the anti-reflection coating.

Examples of optical elements 1 will be explained in the following.

A preferred substrate 10 is a chemically prestressed glass, inparticular in the form of a chemically prestressed glass panel. FIG. 7shows such an example. On the surfaces 100, 101 of both sides, thepanel-shaped substrate 10 has exchange layers 11, 12, which are placedunder compressive stress by exchange of alkali ions of the glass bylarger homologs (in particular, by an exchange of Na+ ions by K+ ions).The surfaces 100, 101 of the two sides form the surfaces that areprovided with the anti-reflection coating 2. Alternatively, only one ofthe side surfaces 100, 101 may also be provided with an anti-reflectioncoating 2, depending on the targeted use.

Likewise, as in the case of the examples shown in FIGS. 1 and 2, it mayalso be favorable in the case of a chemically prestressed substrate 10to deposit a layer 28 containing silicon oxide as an adhesion promoterfor the subsequently deposited anti-reflection coating 2. This layer 28preferably has the same composition as the layers of the anti-reflectioncoating 2 having the first refractive index.

The optical element 1 may be used, for example, as a closing element orwindow of the optics of a camera or of another optical sensor. Theoptical element may also be a cover glass for an optical display of amobile electronic device, such as, for example, a smart phone, a tabletPC or a watch.

In addition, sapphire or Al₂O₃ can also be used as substrate 10 for suchdisplays and can be provided with the coating according to the inventionwith anti-reflection coating 2 and organofluoro layer 3. Here also, ashas already been mentioned above, preferably a layer 28 containingsilicon oxide is used as adhesion promoter between sapphire surface andanti-reflection coating 2. The advantages are the same as in theembodiment of a sapphire watch glass, as explained above.

According to yet another embodiment of the invention, a substrate 10composed of or containing zirconium oxide ZrO₂, is used. A ZrO₂ mixedcrystal is used for such a substrate in order to stabilize a crystalphase, such as the cubic phase. In this case, for example, calciumoxide, magnesium oxide, or yttrium oxide is contained therein asstabilizer.

ZrO₂ has a high modulus of elasticity of approximately 200 GPa. In fact,the flexural rigidity is even higher than in the case of sapphire. Inthis respect, this material is also considered for applications in whicha high substrate strength matters. Therefore, scratch resistancessimilar to sapphire are also obtained with the coating according to theinvention. Disruptive reflections are also suppressed due to the veryhigh refractive index of more than 2.

Additional possible substrates containing zirconium are hard materialssuch as zirconium carbide and zirconium nitride. Of course, thesematerials generally are not transparent or not very transparent. Yetanother possible hard substrate material is silicon carbide, which isalso used as a material in optics.

Instead of a cover glass for optics, a lens may also be directlyproduced as optical element 1 with the coating according to theinvention. FIG. 8 shows such an example. In the example shown, thesubstrate 10 in the form of a lens 8 is provided with theanti-reflection coating 2 and the organofluoro layer 3 applied thereononly on one side. This is useful, for example, if the lens 8 shall becemented to another lens on the uncoated lens surface. Of course, herealso, however, both lens surfaces may also be coated. For substrate 10,preferably optical glasses are used, such as, e.g., crown glasses orflint glasses or highly refractive glasses.

Advantageously, such lenses 8 with layer 3 pointing outward may form anobjective or ocular lens of an objective, for example, for cameraoptics, for a microscope, or for a telescope.

What is claimed is:
 1. An optical element with high scratch resistance,comprising: a substrate that is transparent in the visible spectralregion; a multilayer anti-reflection coating on the substrate, themultilayer anti-reflection coating having alternating layers of a firstlayer type having a first refractive index and layers of a second layertype having a second, higher refractive index, the layers of the secondlayer type comprising nitride or oxynitride, the layers of the firstlayer type comprising oxide of silicon and at least one other element,the first layer type having a molar fraction of silicon that ispredominant when compared to a molar fraction of the at least one otherelement, wherein the multilayer anti-reflection coating has an uppermostlayer that comprises the first layer type; and a layer of chain-formorganofluoro molecules on the multilayer anti-reflection coating, thelayer having molecules that are bonded at their ends to the uppermostlayer of the multilayer anti-reflection coating.
 2. The optical elementaccording to claim 1, wherein the layer of chain-form organofluoromolecules is a monomolecular layer.
 3. The optical element according toclaim 1, wherein the layer of chain-form organofluoro molecules comprisemolecules with perfluorinated carbon chains.
 4. The optical elementaccording to claim 1, wherein the at least one other element of thefirst layer type is an element selected from the group consisting ofaluminum, tin, magnesium, phosphorus, cerium, zirconium, titanium,barium, strontium, niobium, and boron.
 5. The optical element accordingto claim 1, wherein the substrate comprises a sapphire substrate.
 6. Theoptical element according to claim 1, wherein the substrate comprises achemically prestressed glass substrate.
 7. The optical element accordingto claim 1, wherein the multilayer anti-reflection coating comprises astack of four successive layers having a lowermost layer that comprisesa layer of the second layer type, the uppermost layer having asecond-greatest layer thickness among the stack, and wherein themultilayer anti-reflection coating has a lowermost layer that comprisesthe second layer type, the lowermost layer having a layer thickness thatis thinner than the layer thickness of the uppermost layer.
 8. Theoptical element according to claim 7, wherein the at least one otherelement of the layers of the first layer type comprises aluminum.
 9. Theoptical element according to claim 1, wherein the uppermost layer has agreatest layer thickness of the multilayer anti-reflection coating andhas a layer thickness in the range of 100 nm to 700 nm.
 10. The opticalelement according to claim 9, wherein the layer thickness is in therange of 400 nm to 500 nm.
 11. The optical element according to claim 1,wherein the first layer type has a ratio of silicon to the at least oneother element of at least 5:1.
 12. The optical element according toclaim 1, wherein the first layer type has a ratio of silicon to the atleast one other element of at least 8:1.
 13. The optical elementaccording to claim 1, further comprising a Knoop hardness of at least600 kilograms per square millimeter.
 14. The optical element accordingto claim 1, wherein the substrate has a modulus of elasticity of atleast 50 GPa.
 15. The optical element according to claim 1, wherein themultilayer anti-reflection coating comprises a stack of four successivelayers comprising two of the layers of the first layer type and two ofthe layers of the second layer type, an upper of the two layers of thesecond layer having a layer thickness that amounts to at least 40% of athickness of the multilayer anti-reflection coating.
 16. The opticalelement according to claim 15, wherein the upper of the two layers ofthe second layer has a layer thickness that amounts to at least 70% ofthe thickness.
 17. A method for producing an optical element,comprising: preparing a substrate that is transparent in the visiblespectral region; vacuum depositing a multilayer anti-reflection coatingonto the substrate by alternating layers of a first layer type having afirst refractive index and layers of a second layer type having asecond, higher refractive index, wherein the layers of the second layertype comprise nitride or oxynitride and the layers of the first layertype comprise oxide of silicon and at least one other element, whereinthe layers of the first type have a molar fraction of silicon that ispredominant when compared to molar fraction(s) of the at least one otherelement, and wherein the multilayer anti-reflection coating has anuppermost layer that comprises the layer of the first layer type; andapplying a coating with a layer of chain-form organofluoro molecules onthe multilayer anti-reflection coating so that molecules of the layerare bonded at their ends to the uppermost layer.
 18. The methodaccording to claim 17, wherein the step of vacuum depositing comprisesreactive magnetron sputtering.
 19. The method according to claim 17,wherein the step of applying the coating comprises liquid coating. 20.The method according to claim 17, wherein the step of applying thecoating comprises vaporizing in a vacuum deposition method.